Normal monocytes, keratinocytes and lymphocytes as well as cell lines when activated by antigenic or polyclonal stimulants produce a multiplicity of immunoregulatory mediators with potent biological effects on a wide variety of target cells at concentrations of 10 to the minus 10 to 10 to the minus fifteen M. Activated macrophages produce interleukin 1 (IL 1) which enhances the proliferation of peanut nonagglutinating (PNA) thymocytes and induces them to produce the lymphokine IL 2 which in turn induces proliferation by PNA+ thymocytes. The IL 1 has pleomorphic effects in that it stimulates hepatocytes to produce serum amyloid A (SAA), is a growth factor for fibroblasts and has endogenous pyrogenic activity. The keratinocyte factor which is biochemically similar to IL 1, also augments lymphocyte and fibroblast proliferation, as well as IL 2 and SAA production. Murine monoclonal anti human IL 2 antibodies have been obtained which inhibit the T cell growth activity of IL 2 as well as lectin induced lymphocyte proliferation and B cell antibody production in response to antigens. Immunoaffinity columns prepared from these antibodies absorb supernatant IL 2 activity. Studies of these hormone-like factors and inhibitory antibodies will enable us to learn to manipulate immunological and inflammator reactions.